Cold spray is a relatively new technology that has many advantages over conventional thermal spray processes. It is a solid-state coating and material deposition process in which small solid particles are accelerated to high velocities (e.g. 300 to 1200 m/s) by a supersonic or sub-supersonic jet flow through a de Laval nozzle and are subsequently deposited onto the substrate through an impact process to form a coating or deposition. When the high velocity particles impact on a substrate, severe plastic deformation occurs in the particles as well as in the substrate and a deposit is formed. Helium, argon, air, nitrogen, steam, hydrogen or their mixtures are usually used as the processing gas.
There exists a particle critical impact velocity (a threshold) for successful deposition of a given coating and substrate material combination. Only above this critical velocity, the particles can be successfully deposited to form a coating. This critical velocity depends on the properties of the substrate material and its surface conditions, the properties of the coating material, the powder quality, the particle size and the particle impact temperature. High inlet gas pressure and high inlet gas temperature can increase the gas jet velocity and therefore particle velocity. In addition, helium may be used to increase the gas and particle velocity.
As compared to the conventional thermal spray techniques, one of the most important features of the cold spray process is that the temperature of the sprayed particles is always below their melting points and all of the particles are substantially in the solid-state throughout the spray process. This solid-state processing brings several unique advantages, such as avoiding the undesirable chemical change (such as oxidation) and microstructural change (such as grain growth) during the deposition process and producing minimal or even compressive residual stresses. Therefore, cold spray is ideally suitable for depositing oxygen-sensitive materials (e.g. Al, Mg, Ti, Cu, etc.), temperature-sensitive materials (e.g. nano-structured and amorphous materials) and phase-sensitive material (e.g. carbide composites).
Cold spray has the potential to deposit coatings and/or to build three dimensional structures with high deposition rate, very high purity, high density and many other unique properties. This technology has received more and more interests worldwide from both academia and industry.
Currently, based on how powder is introduced into the jet flow, there are two types of cold spray techniques: upstream powder feeding technique and downstream powder feeding technique. The upstream powder feeding technique uses high pressure gases and has high deposition efficiencies but is very expensive from the point of view of equipment and operational cost. On the other hand, the downstream powder feeding technique uses low pressure gas supplies and is portable and much less expensive. However, due to the low particle velocities that can be reached, the downstream powder feeding technique can only deposit limited number of materials and the deposition efficiencies are much lower.
Upstream Powder Feeding Technique:
In the upstream powder feeding system, powder is introduced into the gas flow at the converging section of the nozzle co-axially along with the nozzle central line. The basic structure of an upstream powder feeding cold spray system is described in U.S. Pat. No. 5,302,414 [1].
One of the common problems encountered in the operation of the upstream powder feeding systems is clogging of the nozzle, especially at the nozzle throat between the converging and diverging sections. Steenkiste et al. [4] disclosed a method to mitigate the problem of nozzle clogging during kinetic spray. It was suggested that a second particle population with either a larger average particle diameter or higher yield strength (hardness/elastic modulus) be mixed with the (first) particle population that is to be deposited. The mixture of the particles is accelerated such that the first particle population reaches a velocity that exceeds its critical velocity and is deposited on the substrate to form the desired coating, while the velocity of the second particle population is insufficient to cause its adherence on the substrate and/or the inside of the nozzle, thus avoiding nozzle clogging. In European Patent Publication 1630253, Zhao et al. [5] also suggested to incorporate an additional hard component into the spraying powder in order to prevent nozzle clogging.
In US Patent Publication No. US2005/0214474 [6], Han et al. disclose a nozzle design method for kinetic spray where a gas/powder conditioning chamber is inserted between the prior art powder/gas mixing chamber and the de Laval nozzle. This design was claimed to considerably increase the residence time of particles in the hot main stream gas, increasing the particle temperature and hence the deposition efficiency. Three (upstream) powder injection methods were suggested: (a) co-axial (in-line) with the axis of the conditioning chamber (the conventional method); (b) vertical (oblique) to the axis of the conditioning chamber; and (c) tangential (swirl) to the circumference of the cross-section of the conditioning chamber. Nevertheless, although methods (b) and (c) potentially increase particle temperatures, particle velocity at nozzle exit is lower than in the case of the (conventional) axial injection (method (a)).
US Patent Publication No. 2006/0201418 by Ko et al. [7] teaches the design of nozzle cross-section profiles (along the gas flow direction) and powder injection configurations to reduce/eliminate choking of nozzle throat. According to this disclosure, powder injection tube end is located coaxially in the throat region or in the outlet region beyond the throat area (the conventional diverging section). The proposed nozzle profiles along its axial cross-section do not have the conventional de Laval nozzle shapes.
Downstream Powder Feeding Technique:
In the downstream powder feeding system, the spray powder particles are introduced to the gas flow at a location down the stream after the throat of the nozzle (diverging portion). This configuration eliminates the need for an expensive high-pressure powder feeder. It also avoids the severe wear of the nozzle throat as occurs in the upstream feeding system, thus significantly simplifying the structure of the equipment. Because the spray powder particles are fed through the side of the nozzle in the diverging section where the gas temperature drops dramatically due to the volume expansion, inlet gas temperatures higher than the upstream powder feeding may be permitted to pre-heat the main working gas for increasing gas and particle velocities.
In U.S. Pat. No. 6,402,050 [8], Kashirin et al. discloses an apparatus for cold spray in which the outlet of a low-pressure powder feeder is connected to the diverging section of the nozzle through a conduit. Powder particles are injected radially into the gas stream in the nozzle by atmospheric air flow. In order to effectively transport the powder to the supersonic gas stream, the location for introducing the powder to the nozzle must be determined such that the cross-sectional areas of the supersonic nozzle at the juncture of the nozzle and the powder-feeder conduit is related to the throat area and the gas pressure at the nozzle inlet by a particular relationship.
In U.S. Pat. No. 7,143,967 [9], Heinrich et al. teaches a method for the introduction of spray powder particles co-axially and centrally within the de Laval nozzle not before the convergent section of the nozzle. In this configuration, the tube/nozzle carrying the powder particles passes through the pre-mixing and mixing chambers and the converging regions of the nozzle. US Patent Publication No. 2005/0040260 [10] by Zhao et al. also suggests the use of a coaxial powder injector tube passing through the throat into the diverging section of the nozzle. In this case, the injector tube passes through the centre hole of a gas collimator with surrounding holes. A low pressure powder feeder can be used for these powder feeding configurations.
US Patent Publication No. 2004/0058064 [11] and U.S. Pat. No. 7,108,893 [12], U.S. Pat. No. 6,811,812 [13] and U.S. Pat. No. 6,872,427 [14] disclose and apply a nozzle design method for kinetic spray and spray powder particles are introduced through one or more of a plurality of powder injection inlets located along the diverging section of the nozzle. The use of multiple powder injection points provides a means for more versatile adjusting and control of the spray process and coating quality.
U.S. Pat. No. 6,759,085 awarded to Muehlberger [15] describes a cold spray system that encloses the exit of the accelerated gas and particles from the spray nozzle inside a chamber where the pressure is controlled to much less than atmospheric pressure. This reduced ambient pressure results in substantially higher acceleration of gas and particles under similar static input gas pressures as compared with spraying in normal atmospheric pressures. In addition, an arrangement of valves and powder injection points is provided at various locations along the heating coil and within the spray nozzle to enable powder to be introduced at different selected locations. In this manner, heating of the powder and of the gas can be varied relative to each other to achieve various results.
The Japanese Patent Publication No. 2005-095886 [16] discloses a cold spray nozzle design methodology where the nozzle comprises a short divergent cone portion and a parallel wall extension portion. Powder with or without pre-heating is injected in the parallel extension tube portion at either single or plural locations/points. A low pressure powder feeder can be used.
In US Patent Publication No. 2007/0160769 [17], Maev et al. provide a cold spray gun that continuously measures the powder flow rate using a sensor and, based on this feed back information, either or both of the conveyance gas flow rate and the powder feed rate is/are adjusted so that a stable operation conditions are maintained. The application also suggests the use of axially-spaced multi-injection points for powder feeding downstream the gas flow for the convenience of changing feeding conditions.
In order to prevent adhesion/deposition of particles on nozzle walls, Shkodkin [18] proposed to apply cooling at the diverging portion of the nozzle for downstream powder feeding system. Haynes and Sanders [19] provided a method for preventing nozzle clogging in US Patent Publication No. US2004/0191449 by using polybenzimidazole (PBI) to fabricate at least the diverging portion of the nozzle. This material was found to have good properties of anti-adhesion by spray metal powders.
Polovtsev [31] and Kashirin et al. [32] disclose cold spray devices having two powder injectors where the powder feed injectors are disposed in the downstream portion of the nozzle opposite each other in the same cross section of the nozzle. The injectors are at an angle of 30° to 90° (31] or at angle of 90° [32] to the nozzle axis and the direction of flow. Such a paired powder injection arrangement helps the powder flow substantially along the nozzle centerline and hence reduces the probability of particle deposition/adhesion on the nozzle wall, thereby reducing nozzle clogging.
Comparison to Thermal Spray
In terms of gas and particle acceleration, there are certain similarities in apparatus configurations between the cold spray process and high velocity oxy-fuel (HVOF) thermal spray process. In modern HVOF systems (e.g. Thorpe et al. [22]), a combustion process generates high temperature and high pressure gas flow in a combustion chamber which exits through a nozzle/barrel that may have a de Laval type of shape including a converging and a diverging section. Powder materials are introduced to the hot gas flow in two major configurations: (a) axial powder injection and (b) radial powder injection. The powder is heated to either above or below its melting temperature. The radial powder injection configuration is mainly used in the high-pressure, kerosene-fueled systems. Multiple powder injectors are usually used in this configuration distributed around the circumference of the nozzle downstream of the combustor exit.
Limitations of Existing Technology
In cold spray, the supersonic jet flow is generated through a de Laval type nozzle. For upstream powder feeding cold spray systems, particles are injected axially into the flow at the inlet of the nozzle (upstream of the nozzle throat). Therefore, one of the drawbacks of this type of systems is that a high-pressure powder feeder has to be used running at a gas pressure higher than that in the main gas stream in order to avoid powder back flow. The high-pressure powder feeders are usually very bulky and are much more expensive (over ten times) than the currently commercially available low pressure powder feeders. Another major difficulty associated with these prior art upstream systems is that the de Laval nozzle always has very narrow throat that is prone to clog easily. Clogging becomes much more severe as the particle velocity and temperature are increased. Each combination of particle and nozzle material has a threshold critical velocity and temperature above which the particles will start to block the nozzle. For example, the critical temperature for spraying aluminum using a steel nozzle is approximately 290° C. and is approximately 200° C. for spraying tin using a steel nozzle. Therefore, the inlet gas temperature in the upstream system has to be restricted to certain level to avoid the overheating of powders, the clogging of the powder injector, and the clogging of the nozzle throat. Another drawback associated with the upstream system is the severe wear of nozzle throat due to particle erosion, which affects/modifies the nozzle operation conditions and leads to large variations in operating conditions and deposit quality. This is increasingly problematic when hard particles are being sprayed.
The methods proposed by Steenkiste et al. [4] and Zhao et al. [5] to incorporate a second population of either different material or different particle size into the spray powder mixture to prevent nozzle clogging are practically not feasible. First of all, while the introduction of hard particles may prevent nozzle clogging, it significantly accelerates the nozzle wear. On the other hand, although the second population particles may not reach their critical conditions for forming deposit themselves, i.e., the very hard particles will not deform plastically while large particles (either soft or hard) will not reach their critical plastic deformation velocity required to form deposit on the substrate by themselves, these second population particles will get trapped and enclosed in the deposit/coating by the surrounding first population particles. As a result, a composite coating rather than a coating containing the only intended material of the first population particle will be obtained. This has been shown by the many published results, e.g. in [20], where metal matrix composite coatings are formed by spraying mixtures of metal and hard ceramic particles, although the hard ceramic particles can not deform and form deposit themselves.
The downstream powder feeding cold spray systems introduce the particles into the diverging portion of the nozzle (downstream of the nozzle throat) radially, which eliminates the need for complicated high-pressure powder feeders and thus significantly simplify the equipment. However, there are several shortcomings associated with the current designs of downstream systems. For example, current commercial downstream powder feeding cold spray systems are based on the teaching of U.S. Pat. No. 6,402,050 [8], where powder feeding relies on atmospheric pressure and the siphon effect of the main gas flow in the nozzle. To get adequate powder feeding, the location of powder injection on the nozzle must be coordinated with the inlet gas pressure and the nozzle design is restricted to relatively low exit Mach number (usually <3). Variations on the operating parameters are thus limited once the nozzle design is determined. In addition, there is a maximum inlet gas pressure (normally <1 MPa) that such systems can use, over which the atmospheric pressure will no longer be able to supply powders into the nozzle. As a result, only relatively low particle velocities can be reached through the downstream powder feeding technique.
In US Patent Publication No. 2004/0058064 [11] and U.S. Pat. No. 7,108,893 [12], U.S. Pat. No. 6,811,812 [13], U.S. Pat. No. 6,759,085 [15], and U.S. Pat. No. 6,872,427 [14], use of commercially available powder feeders has been suggested. Proper design, downstream powder feeding systems with pressurized powder feeders potentially allows the use of increased main gas pressures and temperatures and leads to significantly improved particle velocities while still maintaining the advantages of low cost and portability. However, no relationships have so far been defined in the prior art in coordinating pressurized powder feeding with the other operation parameters such as inlet gas pressure and the configurations of the nozzle. Without clear understanding of relationships among all the operating parameters, a stable cold spray process cannot be created. The concept of using multiple powder injection points along the nozzle length provided in the US Patent Publications No. 2004/0058064 [11] and No. 2007/0160769 [17] and U.S. Pat. No. 7,108,893 [12], U.S. Pat. No. 6,811,812 [13], U.S. Pat. No. 6,759,085 [15], and U.S. Pat. No. 6,872,427 [14] does offer increased flexibility over conventional designs.
However, all the methods for the use of a powder feeder to introduce powder particles radially also cause sidewall erosion of the nozzle opposite the point of powder introduction, especially when hard materials are sprayed [10]. In some cases, the edges of the spray path produced by this method are saw-toothed. When relatively soft materials are sprayed or when inadequate processing parameters such as too high processing temperatures and/or pressures are used, adhesion/deposition of the spray materials on sidewalls of the nozzle occurs.
In the powder feeding configurations proposed by Heinrich et al. [9] and Zhao et al. [10], particles are co-axially injected to the downstream gas flow with an injector tube passing through the nozzle throat. At the end of the powder injector tube, there is a sudden change in effective main gas flow cross-section area, i.e., a sudden change in the Mach number. This can lead to considerable gas flow disturbance. Meanwhile, the gap becomes very narrow at the throat, especially for relatively small throat areas and not small enough injector tubes. For example, to obtain a throat cross-section area equivalent to a throat diameter of 2 mm in the conventional nozzle configurations with an injector tube outer diameter of 2 mm, the gap between the injector tube and the nozzle throat is only 0.4 mm. Considerable gas flow friction will occur through such narrow gaps. In addition, any slight misalignment will result in huge imbalance/disturbance in the downstream gas flow. It may even become impossible to maintain stable supersonic flow. US Patent Publication No. 2006/0201418 by Ko et al. [7] has the similar shortcomings and limitations.
Using PBI to fabricate nozzle as proposed in US Patent Publication No. US2004/0191449 [19] may alleviate the nozzle clogging problem; however, the upper working temperature of the material is only 240-400° C. and its wear resistance is not good enough for stable practical applications.
While the apparatuses of Polovtsev [31] and Kashirin et al. [32] address the nozzle clogging problem, another problem arises. In experiments conducted by the present Applicants, it has been found that the cross-sectional area (related to inner diameter) of each powder injector at the junction with the nozzle inner wall must be small enough so that substantial turbulent flow or even shock waves in the main gas flow are prevented. Otherwise, significant reduction of particle velocity and deposition efficiency may occur. However, powder injectors with such small cross-sectional areas are found to be very prone to blocking/jamming by powders, especially at startup and shutdown of the spray operation. The problem becomes more severe at increased working gas pressures. Jamming occurred at every system startup and shutting down.
With regard to HVOF there are significant differences between downstream cold spray processes and HVOF. In the HVOF process, solid particles are heated to considerably higher temperatures (molten or partly molten state) than in the cold spray process. Particle heating is a much more important consideration in HVOF than in the cold spray process, although heating of particles can also be beneficial in cold spray. In cold spray, the major purpose of heating the gas is to increase gas and particle velocities rather than to melt or partially melt the particles as in HVOF. The nozzle diameters in HVOF systems are generally much larger (about 8 mm, for example) [23] than those used in the cold spray process which are generally in the order of 2-4 mm. Thus, clogging and nozzle erosion is generally not a major issue in the HVOF process. The major consideration for using multi-port powder injection in the radial injection HVOF system is to achieve more uniform powder loading in the exit hot gas stream, more efficient use of the available heat and hence substantially higher spray rates than the axial powder injection version of HVOF. There is no teaching in the prior art of HVOF regarding the elimination of nozzle clogging and erosion or the promotion of better gas flow patterns within the nozzle by using such radial powder injection configurations. Further, since HVOF is based on the combustion of an oxy fuel to provide pressurized gas, relationships between the pressure of the gas and the injection pressure of the powder are not the same. Thus, it would not be apparent that configurations successfully used in HVOF systems would be applicable to downstream cold spray systems.
It is thus apparent that the prior art downstream powder feeding cold spray systems are not satisfactory in achieving high particle velocities to produce reliable and consistent deposition with high quality. Further improvement is necessary. It is thus highly desirable to develop an improved cold spray technology that incorporates the advantages of both existing techniques but avoids their disadvantages. There is a need for cold spray technology having deposition capabilities similar to high pressure upstream techniques while being portable, less expensive and easy to maintain and operate.